PATH is what is called a "regional rapid transit system" in that it doesn't mainly serve one particular city, but is a service to nearby areas.
PATH is an electric railway operated by The Port Authority of New York and New Jersey. It operates between Hoboken, Jersey City and Newark, N.J. across the Hudson River to New York City. It is very much like a subway. It runs in tunnels. It uses a third rail. It employs flat fares and farecard options. But it has distinguishing features.
For one thing, it is mandated by the FRA(Federal Railroad Agency), something only Amtrak and commuter railroads, not subway systems, have to deal with. The main reason for this being that it runs on Amtrak's Northeast Corridor for part of it's route(between Newark and Harrison stations).
PATH uses cars similar to subway cars as well(closest to New York City's IRT rolling stock). The cars, which are 51 feet, 8 inches long and 9 feet wide, use overrunning third rail and run on standard gauge track.
PATH(which was originally known as the Hudson and Manhattan Railway) serves several areas with 13 stations and 4 "branches"(map):
The first is the Newark Branch, which runs out of Newark Penn Station. This is the only portion of the system above ground.
Next is the Downtown Branch, which leads directly to the lower level of New York's massive World Trade Center. Here, connections can be made to the New York City subway system.
The Hoboken Branch serves Hoboken Station(NJT) and the fourth branch, which runs along Sixth Avenue, is a major route.
The Sixth Avenue Branch was actually built around and under by the IND's Sixth Avenue subway, a great engineering feat due to the presence of tunnels at Broadway, 34th and 33rd Streets. Along this branch are the system's two abandoned stations, at 19th and 28th Streets.
PATH plays a major role in the transit environment of New York City, Newark and Jersey City. With numerous connections, useful destinations and frequent service, it's not hard to envision it's continued success.